In LTE (Long Term Evolution) of 3GPP (3rd Generation Partnership Project) which is discussed as a next generation mobile communication method, OFDM (Orthogonal Frequency Division Multiplexing) can be employed as a wireless access method of a down link (DL) in the direction from a base station (BS) to a user equipment (UE). As a wireless access method of an up link (UL) in the opposite direction, an SC-FDMA (Single Carrier-Frequency Division Multiple Access) method can be employed.
In these methods, communication can be performed in a manner that signals to a plurality of user equipment (simply referred to as “users” in some cases, hereinafter) or signals from a plurality of users are multiplexed into mutually different frequency bands at the same time (timing).
Further, such discussed methods include: a single user MIMO (Multi-Input Multi-Output) method in which independent signals transmitted from a plurality of antennas are received through a plurality of receiving antennas; and a multi-user MIMO method in which transmission and receiving are performed simultaneously between a plurality of antennas of a base station and a plurality of user equipment.
Here, with taking into consideration the throughput, the coverage, and the like, a scheduler in the base station can perform scheduling in a time domain, a frequency domain, or a space domain (between transmission antennas). That is, with recognizing the receiving condition and the like of users distributed in the radio area (cell) of a base station, the scheduler can allocate the radio resources (such as the frequency and the timing) to the users, and then instruct the user terminals through the control channel to perform transmission operation of UL data and receiving operation of DL data.
Algorithms used in scheduling include a Round Robin (RR) method, a Maximum Carrier-to-Interference power Ratio (MaxCIR) method, and a Proportional Fairness (PF) method.
In round robin scheduling, transmission and receiving opportunities are allocated to individual users sequentially (without setting up priority).
In the MaxCIR method, transmission and receiving opportunities are allocated with priority to users having the highest channel quality (receiving quality). The MaxCIR method is an algorithm preferable from the viewpoint of maximizing the throughput (system throughput) of the entire system. Nevertheless, a possibility is present that transmission and receiving opportunities are seldom allocated to users having poor receiving quality.
Thus, the PF method has been devised from the viewpoint of allocating transmission and receiving opportunities fairly to individual users and yet maximizing the system throughput. In the PF method, a value obtained by dividing the instantaneous rate (throughput) by the average rate (throughput) is adopted as the priority of each user terminal, and then transmission and receiving opportunities are allocated with priority to users having the maximum priority having been calculated.
An example of a calculation formula for the priority (ρk,l) in the PF method is shown as the following Formula (1). Here, k is the index of a user, and l is the index of time (timing). Further, rk,l denotes the instantaneous receive ready rate of the user #k at the timing #l. This quantity is obtained on the basis of the SIR (Signal to Interference Ratio) and the performance of the receiver. Further, Rk denotes the average receive rate of the user #k, that is, the average rate that the user #k has actually succeeded in receiving.
                              ρ                      k            ,            l                          =                              r                          k              ,              l                                            R            k                                              (        1        )            
The above-mentioned PF method can be employed in frequency scheduling. That is, the SIR (or prediction throughput) of each user is calculated for each of a plurality of frequency bands (identified by an index i). Then, for each frequency band i, a priority (ρi, k, l) is calculated on the basis of a PF method, for example, shown by the following Formula (2). Then, transmission and receiving opportunities are allocated to users having the highest priority.
                              ρ                      i            ,            k            ,            l                          =                              r                          i              ,              k              ,              l                                            R            k                                              (        2        )            
By using an arbitrary weight coefficient α, this Formula (2) can be generalized as shown by the following Formula (3).
                              ρ                      i            ,            k            ,            l                          =                              r                          i              ,              k              ,              l                                            R            k            α                                              (        3        )            
Here, when α=0, the scheduling corresponds to a MaxCIR method. When α=1, the scheduling corresponds to a PF method. When α→˜, the scheduling becomes such that the average throughputs of the individual users are equalized (a higher priority is imparted to a user having a lower average throughput).
The following description is given for the scheduling of the UL. However, similar discussion holds for the scheduling of the DL.
Transmitted radio waves of a user (referred to as a cell edge user, hereinafter) located on the boundary of a cell (or sector) or its vicinity (referred to as a cell edge, hereinafter) interfere with those of cell edge users of adjacent cells in some cases. Thus, when transmission opportunities are allocated simultaneously to cell edge users of individual adjacent cells, the cell edge throughputs decrease substantially in some cases.
Thus, in order to avoid this situation and improve the cell coverage, an approach is proposed that bands for cell edge users and bands for users (cell center users) located near a base station are set up concerning the frequency bands, and that the band setting is changed (made different) between adjacent cells.
For example, a threshold value is set up concerning the receiving SIR of the individual users. Users having a receiving SIR value higher than the threshold value are defined as cell center users, while users having a receiving SIR value lower than the threshold value are defined as cell edge users. Then, the system frequency band is divided scheduling units (these divided bands are referred to as subbands), and then correspondence is established between the subbands and the user classes. That is, particular subbands are dedicated to cell center users, while other particular subbands are dedicated to cell edge users.
This correspondence is shifted (made different) for individual subbands such that cell edge users in adjacent cells (or sectors) may not obtain transmission opportunities simultaneously in the same subband. This improves the throughputs of cell edge users.
On the other hand, in the UL multiuser MIMO method, in an UL, MIMO receiving is performed in a state that a plurality of users are assigned to the same time and the same frequency, so that the throughput is improved. The outline of the multiuser MIMO method is described below. The following description is given for a case that the number of transmission antennas of each user is 1. When N user numbers (indices) selected by a scheduler are denoted by k(1), k(2), . . . , k(N), the received signal yk is expressed as shown by the following Formula (4).
                              y          k                =                                                            ∑                                  i                  =                  1                                N                            ⁢                                                h                                      k                    ⁡                                          (                      i                      )                                                                      ⁢                                  x                                      k                    ⁡                                          (                      i                      )                                                                                            +            n                    =                                                    H                k                            ⁢                              x                k                                      +            n                                              (        4        )            
where xk(i) denotes a transmission signal vector from a user k(i) (i=1 to N), hk(i) denotes a channel estimate vector with respect to a user k(i), H denotes a channel matrix, and n denotes a noise vector.
When the number of users per cell (sector) is denoted by Nuser, the number of candidates of selection in scheduling isNuserCN For example, when N=2, the number of candidates of selection is Nuser(Nuser−1)/2.
Further, in order to improve the throughput of the multiuser MIMO method, an MMSE-SIC (Minimum Mean Square Error-Successive Interference Cancel) method is discussed. In the MMSE-SIC method, a plurality of transmission streams are equalized and decoded sequentially by using an MMSE weight. Then, when a CRC (Cyclic Redundancy Check) result is OK, a replica signal is generated on the basis a re-encoded transmission symbol sequence and a channel estimate value, and then this replica signal component is cancelled from the received signal. The sequence of the above-mentioned equalization and decoding in the MMSE-SIC is referred to as a “layer”.
In the UL multiuser MIMO method, when a plurality of cell edge users are multiplexed into the same subband at the same timing, this can cause an increase of interference between the cells, and hence can cause degradation in the throughput. In order to avoid this situation, an approach is proposed that cell edge users and cell center users are distinguished as described above and then assigned to appropriate layers so that the throughput is improved.
There are four documents pertinent to the related art, Japanese Laid-Open Patent Publication No. 2005-236918, Japanese Laid-Open Patent Publication No. 2005-191745, A. Jarali, R. Padovani, R. Pankaj, “Data Throughput of CDMA-HDR a High Efficiency-High Data Rate Personal Communication Wireless System,” Proceeding of IEEE VTC-2000 Spring, and Nortel, 3GPP TSG-RAN WG1#49bis R1-072774 (“UL MU-MIMO Performance Improvement for E-UTRA”), Orlando, Fla. USA, Jun. 25-29, 2007.
Nevertheless, the above-mentioned method that cell edge users and cell center users are distinguished (grouped) on the basis of a threshold value causes fixation of the subbands or layers to which the users of individual groups are to be assigned. This prevents flexible processing for such cases that the distribution of users varies within the cells or sectors, and hence can cause inefficient allocation such as exhaustion of particular radio resources.